To check your Membership Store Credit amount, please login to your account and then click on My Membership listed under My Stuff drop down.
Then on the My Membership page, you can see your Membership Start Date, your Next Payment Date, your Monthly Membership Fee, and your Available Store Credit.
To learn how to apply your Membership Store Credit, please visit this page. Store credit is not automatically applied at checkout. You will need to enter the amount you would like to use towards your order.